1. Field of the Invention
The present invention relates to a rate control system for controlling the rate of a bit stream including a compressed image signal.
2. Description of the Background Art
As a next-generation high-efficiency coding standard for image data, the International Organization for Standardization (ISO) and the International Telecommunications Union-Telecommunication Standardization Sector (ITU-T) have being developing the Joint Photographic Experts Group 2000 (JPEG2000) standard. The JPEG2000 standard provides functions superior to the Joint Photographic Experts Group (JPEG) standard which is currently in the mainstream, and features the adoption of discrete wavelet transform (DWT) for orthogonal transformation and of a technique called “Embedded Block Coding with Optimized Truncation (EBCOT)” which preforms bit-plane coding, for entropy coding.
Hereinbelow, a general description of the procedure of compression and coding according to the JPEG2000 standard is given with reference to a compression encoder 100 shown in FIG. 23.
An image signal inputted to the compression encoder 100 is DC level shifted in a DC level shift unit 102 as needed, and outputted to a color-space conversion unit 103. The color-space conversion unit 103 converts the color space of a signal inputted from the DC level shift unit 102. Then, a tiling unit 104 divides an image signal inputted from the color-space conversion unit 103 into a plurality of rectangular regional components called “tiles” and outputs those components to a DWT unit 105. The DWT unit 105 performs integer or real-number DWT on each tile of an image signal inputted from the tiling unit 104 and outputs transform coefficients as a result. In DWT, a one-dimensional (1-D) filter, which divides a two-dimensional (2-D) image signal into high-pass (high-frequency) and low-pass (low-frequency) components, is applied in vertical and horizontal directions in this order. In the fundamentals of the JPEG2000 standard, an octave band splitting method is adopted in which only those bandpass components (subbands) which are divided into the low frequency side in both the vertical and horizontal directions are recursively divided into further subbands. The number of recursive divisions is called the decomposition level.
FIG. 24 is a schematic view showing a 2-D image 120 subjected to DWT with the third decomposition level using the octave band splitting method. At the first decomposition level, the 2-D image 120 is divided into four subbands HH1, HL1, LH1, and LL1 (not shown) by sequential application of the aforementioned 1-D filter in the vertical and horizontal directions. Here, “H” and “L” stand for high- and low-pass components, respectively. For example, HL1 is the subband consisting of a horizontally high-pass component H and a vertically low-pass component L of the first decomposition level. To generalize the notation, “XYn” (X and Y are either H or L; n is an integer of 1 or more) represents a subband consisting of a horizontal component X and a vertical component Y of the n-th decomposition level.
At the second decomposition level, the low-pass component LL1 is divided into subbands HH2, HL2, LH2, and LL2 (not shown). Further, at the third decomposition level, the low-pass component LL2 is divided into further subbands HH3, HL3, LH3, and LL3. An arrangement of the resultant subbands HH1, HL1, LH1, HH2, HL2, LH2, HH3, HL3, LH3, and LL3 is shown in FIG. 24.
A quantization unit 106 has the function of performing scalar quantization on transform coefficients outputted from the DWT unit 105 as needed. The quantization unit 106 also has the function of performing a bit-shift operation in which higher priority is given to the image quality of an ROI (region of interest) which is specified by an ROI unit 107. Now, in reversible (lossless) transformation, scalar quantization is not performed in the quantization unit 106. The JPEG2000 standard provides two kinds of quantization means: the scalar quantization in the quantization unit 106 and post-quantization (truncation) which will be described later.
Then, transform coefficients outputted from the quantization unit 106 are, according to the aforementioned EBCOT, entropy coded on a block-by-block basis in a coefficient bit modeling unit 108 and an arithmetic coding unit 109, and they are rate controlled in a rate control unit 110. More specifically, the coefficient bit modeling unit 108 divides each subband of input transform coefficients into regions called “code blocks” of, for example, approximately size 32×32 or 64×64 and further decomposes each code block into a plurality of bit planes each constituting a two-dimensional array of respective one bits of the transform coefficients.
FIG. 25 is a schematic view showing the 2-D image 120 decomposed into a plurality of code blocks 121. FIG. 26 is a schematic view showing n bit planes 1220 through 122n−1 (n is a natural number) consisting of each code block 121. As shown in FIG. 26, decomposition is performed such that, where a binary value 123 representing one transform coefficient in a code block 121 is “011 . . . 0,” then bits constituting this binary value 123 belong respectively to the bit planes 122n−1, 122n−2, 122n−3, . . . , and 1220. In the figure, the bit plane 122n-1 represents the most-significant bit plane consisting only of the most-significant bits of the transform coefficients, and the bit plane 1220 represents the least-significant bit plane consisting only of the least-significant bits of the transform coefficients.
Then, the coefficient bit modeling unit 108 judges the context of each bit in each bit plane 122k (k=0 to n−1), and as shown in FIG. 27, decomposes the bit plane 122k according to the significance of each bit judgment result), into three types of coding passes: a significance propagation (SIG) pass, a magnitude refinement (MR) pass, and a cleanup (CL) pass. The context judgment algorithm for each coding pass is determined by the JPEG2000 standard. According to the algorithm, the state of being “significant” means that a coefficient concerned has already been found not to be zero in previous coding, and the state of being “not significant” means that the value of a coefficient concerned is or possibly zero.
The coefficient bit modeling unit 108 performs bit-plane coding with the three types of coding passes: the SIG pass (coding pass for insignificant coefficients with significant neighbors), the MR pass (coding pass for significant coefficients), and the CL pass (coding pass for the remaining coefficients which belongs to neither the SIG nor MR pass). The bit-plane coding is performed, starting from the most-significant to the least-significant bit plane, by scanning each bit plane in four bits at a time and determining whether there exist significant coefficients. The number of bit planes consisting only of insignificant coefficients (0 bits) is recorded in a packet header, and actual coding starts from a bit plane where a significant coefficient first appears. The bit plane from which coding starts is coded in only the CL pass, and lower-order bit planes than that bit plane are sequentially coded in the above three types of coding passes.
Then, the arithmetic coding unit 109, using an MQ coder, performs arithmetic coding of a coefficient sequence provided from the coefficient bit modeling unit 108 on a coding-pass-by-coding-pass basis according to the result of context judgment. This arithmetic coding unit 109 also has a mode of performing bypass processing in which a part of the coefficient sequence inputted from the coefficient bit modeling unit 108 is not arithmetically coded.
Then, the rate control unit 110 performs post-quantization for truncation of lower-order bit planes of a code sequence outputted from the arithmetic coding unit 109, thereby to control a final rate. A bit-stream generation unit 111 generates a bit stream by multiplexing a code sequence outputted from the rate control unit 110 and attached information (header information, layer structure, scalability, quantization table, etc.) and outputs it as a compressed image.
The compression encoder with the aforementioned configuration adopts, as a method for compressing the amount of image data, for example a technique called rate-distortion (R-D) optimization utilizing the rate control method employed in the rate control unit 110 (cf. David S. Taubman and Michael W. Marcellin, “JPEG2000 Image Compression Fundamentals, Standards and Practice,” Kluwer Academic Publishers, which is hereinafter referred to as the “non-patent literature”).